In many industrial processes there is a need of one or more measurements of level, i.e. the vertical position of the surface of a fluid; interface level, i.e. the vertical position of the layer between two fluids; and density profile, i.e. the density of fluids as a function of the height measured at discrete points or continuously along a vertical axis; in tanks to enable automatic control of the related operation. For certain processes the measurement may be very hard or impossible to carry out. Some factors that make this measurement difficult can be harsh or inaccessible environments, aggressive or dangerous fluids and/or liquids, high pressure or temperatures, fluids causing deposits inside the tank/container, and so on. One example of a process that involves several of these problems is in sub-sea oil separator tanks. A nucleonic system is one method of measurement in this type of separator tanks.
In the technique previously known one usually uses a vertical array of several gamma radiation sources and a vertical array of several radiation detectors positioned at a suitable distance from each other. The sources and the detectors are normally inserted in two separate protection pipes. The two pipes are mounted either both inside the tank or one inside and the other outside it. The gamma radiation sources are also shielded by lead with openings towards the detectors. Horizontal radiation beams from the gamma sources are pointed at each of the detectors. The attenuation of the radiation beam passing through the actual fluids, i.e. the gas and/or liquids, will vary depending on the amount of gas and the density of the liquid. A higher density will cause more attenuation and lower signals from the detector. In this way the detector signals along the vertical axis represent a function of the density profile. Differences in adjacent detector signals indicate a fluid surface or interface level at that position. Instead of an array of radiation detectors, it is possible to use one long vertical detector according to the present invention having associated electronics that can separate the radiation the different beams. In both cases scintilliation detectors could be used. The array of separate gamma radiation sources could be replaced by one vertical wire source.
The problems when using an array of PTM detectors, i.e. a scintillator with an attached photo multiplier tube, are that the reliability of the system will be reduced and the costs will increase for large tanks or containers because of the fact that a large number of parts will be needed. This also limits the height resolution because of the size of the detectors and the maximum number that is practical. By the expression xe2x80x9cscintillatorxe2x80x9d is meant a material that emits light flashes when exposed to nuclear radiation, preferably gamma radiation. It typically consists of plastic or crystal for industrial applications.
The object of the present invention is to provide an arrangement of the type mentioned in the introduction, whereby the drawbacks existing in previously known technique are eliminated. The characterizing features are set forth in the accompanying claims.
According to the invention there is proposed a solution with a long detector unit comprising a long scintillator rod with associated photo multiplier tubes covering the total height used instead of the array of PMT detectors. The long detector unit has a PMT, i.e. a photo multiplier tube, constituted by a photo detector, which is very sensitive to light, at each end and associated electronics that provides both information on position as well as radiation level at the different positions along the detector unit.